Ink jet printing is a recording mode for printing characters, images or diagrams by jetting liquid inks from nozzles to recording media, using a pressure, heat, an electric field or the like as a drive source. Ink jet printing has found an increasing market, because it is lower in running cost and is capable of providing high image quality, and also because various kinds of inks such as water-based inks, oil-based inks, etc. can be used in accordance with end uses.
Under such circumstances, there have been developed large-scale ink jet printers for use together with water-based inks, capable of printing on absorbing recording media such as paper of A-0 size. These ink jet printers are used for printing in-door posters and graphics by CAD, and printing for proofing color matching prior to printing. Such printed matters are laminated for outdoor use.
In these years, a demand for printed matters for outdoor use has been increasing. To meet such a demand, oil-based inks which can be directly printed on plastic films of polyvinyl chlorides or the like have been developed. There also have been developed such oil-based inks that are excellent in waterproofing and weather resistance and can provide printed matters having sufficient scratch resistance without the need of lamination thereof.
As an ink excellent in scratch resistance, an ink comprising a vinyl chloride resin dissolved in an organic solvent is proposed. As an ink excellent in waterproof and weather resistance, an ink comprising a pigment with high fastness dispersed in an organic solvent is proposed.
However, when these oil-based inks are used with printer heads adjusted for conventional water-based inks, they accelerate the corrosion of nozzle plates. Furthermore, when such oil-based inks are left to remain in the printers for a long period of time, their ink-jet stability degrades, which causes problems in practical use, such as degradation of the quality of printed matters.
Factors of such problems are considered to be the use of organic solvents with a high resin-dissolving power so as to dissolve acrylic resins or vinyl chloride resins, the use of acidic pigments which are surface-treated by oxidation or acidic derivatives, and the contents of acidic substances such as chlorine ions and sulfate ions as impurities in dyes. These factors are considered to facilitate chemical corrosion of nozzle plates.
As techniques for preventing the corrosion of nozzle plates, there are known, for example, an oil-based pigment ink capable of inhibiting corrosion of an aluminum electrode head member by the use of a specific amount of an ethanolamine-based compound (cf. JP-A-2005-23099); an oil-based ink capable of inhibiting corrosion of a metal of a head member due to a galvanic action, by the use of 3-methoxy-3-methyl-butyl acetate and a specific resin (cf. JP-A-2004-262985); an oil-based ink capable of preventing corrosion of an ink-repelling-treated head by the use of a diethylene glycol compound and a dipropylene glycol compound in a given mixing ratio (cf. WO2002/055619); and an oil-based dye ink capable of preventing corrosion of an electrode by decreasing the content of a water-soluble inorganic salt (cf. JP-B2-2734032).
Apart from the above-described techniques for preventing the corrosion, several techniques for improving the stability of oil-based inks are known. For example, there are disclosed an oil-based pigment ink capable of maintaining its dispersion stability even after being left to stand for a long period of time, by the use of an acidic carbon black with pH 2.0 to 4.0 as a colorant, together with a specific solvent (cf. JP-A-2001-220527); an oil-based dye ink capable of inhibiting precipitation of a dye during the storage thereof for a long period of time, by the use of a red dye having a specific structure and an alkylamine ethylene oxide derivative (cf. JP-A-2000-212495); and an oil-based dye ink which is improved in dye-dissolution stability by the use of an oil-soluble dye, a petroleum-based solvent and polyoxyethylene alkylamine (cf. JP-B-8-6057/1996).